1. Field of the Invention
The present invention relates to a power supply circuit for supplying power to an electronic device such as an image forming apparatus.
2. Description of the Related Art
A developing device in an electrophotographic or electrostatic printing image forming apparatus efficiently develops an electrostatic latent image with toner by applying, to a developing sleeve, a voltage in which DC (Direct Current) and AC (Alternating Current) voltages are superposed. In particular, an AC voltage having a rectangular waveform increases the charging efficiency of toner with respect to a latent image (ratio at which toner charges couple with latent image charges).
A voltage applied to the developing sleeve needs to have a target value. This is because various problems arise if an applied voltage greatly exceeds a target voltage to generate an overshoot. For example, a current flows through an unintended conductor via the surface of an insulator or an air layer. Also, aerial discharge occurs in a conductive impurity mixed in a developing agent, damaging a latent image. As one solution to relieve these problems, a damping resistor having a sufficiently large resistance value is adopted.
However, the use of the damping resistor also has disadvantages. For example, the rise and fall responses become slower than those of an ideal rectangular wave, resulting in a blunt rectangular wave. The blunt rectangular wave is poorer in charging efficiency than the ideal rectangular wave. The power loss across the damping resistor accounts for half the input power to an AC voltage generation circuit, increasing the space for permitting the energy loss and raising the component cost.
In Japanese Patent Laid-Open No. 2002-354831, an AC voltage generation circuit is formed from a full bridge circuit made up of four switching elements. A predetermined OFF period is set in part of a period during which each switching element is turned on. This arrangement relieves an overshoot without depending on a damping resistor.
The invention disclosed in Japanese Patent Laid-Open No. 2002-354831 can correct an output waveform distortion by LC resonance satisfactorily for a developing unit, photosensitive member, developing high-voltage power supply, and developing agent under a given condition. However, a transformer connected to the AC voltage generation circuit varies in leakage inductance owing to an individual difference. The magnitude of even a capacitance formed by the gap between the developing sleeve and the photosensitive member varies depending on mounting of the developing sleeve and photosensitive member (gap tolerance). The resonant waveform changes depending on the leakage inductance and load capacitance. In the above invention, the first ON period, OFF period, and second ON period of the switching element need to be set for an individual image forming apparatus. Recently, the gap between the developing sleeve and the photosensitive member tends to be smaller for higher charging efficiency. The capacitance C1 by the gap is given byC1=εS/d 
ε: permittivity
S: facing area between the developing sleeve and the photosensitive member at a portion which contributes to development
d: facing distance between the developing sleeve and the photosensitive member
As long as the variation tolerance Δd does not change, even if the gap distance d decreases, Δd becomes larger than d. Since the capacitance C1 is inversely proportional to the distance d, the variation sensitivity of the capacitance C1 to Δd increases. In addition to the individual difference of the image forming apparatus, new variations may be generated even after shipment. For example, the photosensitive member and developing unit are consumables and are sometimes exchanged. Also, an abutting member which regulates the gap distance d between the developing sleeve and the photosensitive member galls. Further, a member may deform by temperature and humidity. The impedance of a path from the developing high-voltage power supply circuit to the developing sleeve and photosensitive member may change. Under the circumstances, even if the ON and OFF periods of the switching element are adjusted not to generate a distortion upon shipment, the resonant waveform changes over time. Especially, a waveform in an image forming apparatus which omits the damping resistor or decreases the number of damping resistors is higher in sensitivity to a change of the capacitance C1 than a waveform which is blunted sufficiently by the damping resistor. Accordingly, the waveform changes from an ideal rectangular wave, hindering stable development.